During the course of efforts of keeping emissions that are liberated during the operation of internal combustion engines as low as possible, the so-called exhaust gas recirculation plays an important part for improving the thermal efficiency of the internal combustion engine. In principle, a high-pressure exhaust gas recirculation and a low-pressure exhaust gas recirculation are distinguished. In case that the internal combustion engine is supercharged, i.e. the driving device includes a compressor such as a turbocharger, the exhaust gas recirculation can take place in the regions of the feed line and discharge line, in which the combustion air and the exhaust gas are still compressed, i.e. are under a pressure above atmospheric pressure. This constitutes a high-pressure exhaust gas circulation. If the exhaust gas recirculation is recirculated outside these regions, a low-pressure exhaust gas recirculation is present. In case of naturally aspirated internal combustion engines, exclusively a low-pressure exhaust gas recirculation takes place. Such driving devices with a high-pressure exhaust gas recirculation are known from US 2007/0186536 A1 and DE 603 20 574 T2.
In particular for utilizing the high-pressure exhaust gas recirculation a positive pressure gradient is necessary. The pressure of the exhaust gas in the discharge line before the exhaust gas turbine has to be greater than the pressure of the combustion air or of the mixture of combustion air and recirculated exhaust gas in the feed line. Otherwise, the recirculated exhaust gas has to be delivered into the feed line with the help of a delivery unit such as a pump, which requires an additional component and leads to an increase of the fuel consumption.
In case of internal combustion engines in which the combustion air is supercharged by means of a turbocharger, a negative pressure differential is present in the region of low rotational speeds and high loads. In other words, the pressure in the feed line is greater than in the discharge line in front of the exhaust gas turbine. In this region, the high-pressure exhaust gas recirculation can therefore not be employed without an additional delivery unit. Only from rotational speeds of the internal combustion engine of approximately 3,000 rpm does the pressure in the discharge line in front of the exhaust gas turbine become greater than the pressure in the feed line, so that the exhaust gas recirculation can then be employed.
In the region of the rated output, the high-pressure exhaust gas recirculation causes the temperature of the mixture of combustion air and recirculated exhaust gas to significantly rise, which results in clear disadvantages with respect to the fuel consumption.
In particular, in the feed line of internal combustion engines, the combustion air of which is compressed by means of a turbocharger, a charge air cooler is usually arranged in flow direction of the combustion air to the internal combustion engine downstream of the turbocharger in order to again offset the heating of the combustion air caused by the compression among other things for the aforementioned reasons. The aim is to have a combustion air temperature of 35° C. However, the charge air cooler causes a dynamic stagnation pressure and thus a pressure increase upstream of the charge air cooler in the feed line. When the exhaust gas is introduced into the feed line upstream of the charge air cooler the temperature of the mixture of combustion air and recirculated exhaust gas can be lowered but the driving pressure gradient, because of the dynamic stagnation pressure of the charge air cooler, is lower, which is why a high-pressure exhaust gas recirculation can only be employed at significantly higher rotational speeds.